A method of increasing the amount of a melt extruded from a spinneret in melt-spinning for production of polyester fibers is very effective for increasing the output or productivity. At present, the demand for fibers having a finer denier is increasing. However, if the speeds of taking up the spun yarns are the same, the productivity of a fine yarn is lower than that of a thick yarn. The above-described method is very desirable for overcoming the low productivity which has been unavoidable when producing a fine yarn, and for increasing the productivity and reducing the yarn-manufacturing cost. For this purpose, there can be mentioned a method in which the speed of taking up a spun yarn is increased and the amount of a melt extruded from a spinneret is thereby increased. According to this method, however, since the take-up speed is high, the molecular orientation of the spun yarn is generally enhanced, resulting in reduction of the residual elongation of the obtained undrawn yarn. Therefore, at the subsequent drawing or draw texturing step, the draw ratio should naturally be decreased. Accordingly, although the amount of the extruded melt is increased because of the increase of the take-up speed during a melt spinning step, the effect which is less than that previously expected can only be obtained due to the reduction of the draw ratio during a drawing step. More specifically, in melt-spinning of a polyester, when the denier of the intended yarn obtained through a drawing step is denoted by de and the draw ratio in the drawing step is denoted by DR, the denier of the spun yarn is expressed by de xDR. If the speed (m/min) of taking up the spun yarn is denoted by V, the extrusion amount Q (g/min) of the molten polymer is expressed by the following formula: ##EQU2##
Accordingly, if the denier of the drawn yarn is constant, the extrusion amount Q is proportional to the product of the take-up speed V multiplied by the draw ratio DR. If the draw ratio DR is constant regardless of the take-up speed V, the extrusion amount Q is proportionally increased as the take-up speed V is increased. However, in fact, as described above, the draw ratio DR is decreased as the take-up speed is increased, and therefore, the extrusion amount does not increase as it is expected.
This problem will be solved by controlling increase of the molecular orientation of the spun yarn even if the speed of taking up the spun yarn is increased. As such a means, there has been considered a method in which a heated spinning chimney is disposed just below the spinneret, and the stream of the polymer extrudate is gradually cooled and solidified. This method, however, is defective in that a specially designed heating apparatus should be disposed and control of the process of cooling the spun yarn is very difficult.
Separately, there has been proposed a method in which a chain branching agent such as pentaerythritol is copolymerized as a third component with a polyester and the extrusion amount and draw ratio are increased to improve the productivity (see Japanese Patent Application Laid-Open Specification No. 292/78). This method, however, is still disadvantageous in that special polymerization conditions must be adopted; also the spinning condition is often degraded and the physical properties of the obtained yarn are adversely affected.
The present inventors have carefully done research with a view to overcoming the foregoing defects involved in the conventional methods, and they have found that when a polymer having a certain specific recurring unit structure is added to a polyester, even if the take-up speed is increased, the molecular orientation is not enhanced, whereby an undrawn polyester yarn having a high residual elongation is obtained. As a result, drawing at a high draw ratio becomes possible and productivity is increased. The present inventors have now completed the present invention based on this finding.